Add %{branch_creator} template variable for issue branch names

Re-created !246421 (closed) to get into July hackathon.

What does this MR do and why?

Add %{branch_creator} template variable for issue branch names

Threads current_user through Issue#to_branch_name and #suggested_branch_name so the branch name template can include the username of the person creating the branch.

References

Fixes #463264 (closed)

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